Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) Release 8/9 (LTE R8/9) supports up to 100 Mbps in the downlink (DL), and 50 Mbps in the uplink (UL) for a 2×2 configuration. The LTE DL transmission scheme is based on an orthogonal frequency division multiple access (OFDMA) air interface.
For flexible deployment, LTE R8/9 systems support scalable transmission bandwidths, one of 1.4, 2.5, 5, 10, 15, or 20 MHz. In LTE, each radio frame (10 ms) comprises 10 equally sized sub-frames of 1 ms. Each sub-frame comprises 2 equally sized timeslots of 0.5 ms each. Either 7 or 6 orthogonal frequency division multiplex (OFDM) symbols exist per timeslot depending on the length of the cyclic prefix (CP). 7 symbols per timeslot are used with normal CP length, and 6 symbols per timeslot are used with the extended CP length. The sub-carrier spacing for the LTE is 15 kHz. An alternative reduced sub-carrier spacing mode using 7.5 kHz is also possible.
A resource element (RE) corresponds to one (1) sub-carrier during one (1) OFDM symbol interval. 12 consecutive sub-carriers during a 0.5 ms timeslot constitute one (1) resource block (RB). Therefore, with 7 symbols per timeslot, each RB consists of 12×7=84 REs. A DL carrier may comprise a scalable number of RBs, ranging from 6 RBs up to 110 RBs. This corresponds to an overall scalable transmission bandwidth of roughly 1 MHz up to 20 MHz. Normally, a set of common transmission bandwidths is specified, e.g., 1.4, 3, 5, 10, or 20 MHz.
The basic time-domain unit for dynamic scheduling is one sub-frame comprising two consecutive timeslots, which may be referred to as a resource-block pair. Certain sub-carriers on some OFDM symbols are allocated to carry pilot signals in the time-frequency grid. A given number of sub-carriers at the edges of the transmission bandwidth are not transmitted in order to comply with spectral mask requirements.
LTE-Advanced with carrier aggregation is an evolution that aims to improve single carrier LTE R8/9/10 data rates using, among other methods, bandwidth extensions (i.e., carrier aggregation). With carrier aggregation, a WTRU may transmit and receive simultaneously over a physical uplink shared channel (PUSCH) and a physical downlink shared channel (PDSCH), respectively, on multiple serving cells. Up to four secondary cells (SCells) may be configured in addition to a primary serving cell (PCell). It may support flexible bandwidth assignments up to 100 MHz.
The control information for the scheduling of PDSCH and PUSCH may be sent on one or more physical downlink control channels (PDCCHs). In addition to the LTE R8/9 scheduling using one PDCCH for a pair of UL and DL carriers, cross-carrier scheduling may be supported for a given PDCCH, allowing the network to provide PDSCH assignments and/or PUSCH grants for transmissions in other serving cell(s).
In LTE R8/9 and LTE Release 10 (R10) with single carrier configuration, where the network assigns a wireless transmit/receive unit (WTRU) one pair of UL and DL carriers, for any given subframe there is a single hybrid automatic repeat request (HARQ) process active for the UL and a single HARQ process active in the DL.
In LTE R10 with carrier aggregation configured, there is one HARQ entity for each serving cell. There may be more than one HARQ processes active for the UL and for the DL in any given subframe, but at most one UL and one DL HARQ process per configured serving cell.